Cam phasers are used to control the angular relationship of the pulley/sprocket to the camshaft of an engine. A variable cam phaser (VCP) allows the phase relationship to change while the engine is running. Typically, a cam phaser is used to shift the intake cam on a dual overhead cam engine in order to broaden the torque curve of the engine, to increase peak power at high rpm, and to improve the idle quality. Also, the exhaust cam can be shifted by a cam phaser in order to provide internal charge diluent control, which can significantly reduce HC and NOx emissions, or to improve fuel economy.
Cam phasers are controlled by hydraulic systems, which use pressurised lubrication oil from the engine in order to change the relative position between camshaft and crankshaft, by rotating the camshaft towards advance or retard positions, thus altering the valve timing.
To control rotation of the camshaft the cam phaser is provided with two chambers that receive oil: an advance chamber and a retard chamber. To rotate the camshaft in the advance direction, oil is pumped out of the retard chamber and into the advance chamber, and to rotate the camshaft in the retard direction, oil is pumped out of the advance chamber and into the retard chamber.
The flow of oil between the chambers, and hence the rotation of the cam shaft, is generated by the cam shaft torque oscillations and is controlled via an oil control valve (OCV). The OCV typically consists of a housing that has an advance port leading to the advance chamber and a retard port leading to the retard chamber. A spool is movable within the housing to route oil between the ports. The spool has an internal cavity with an oil port that receives oil from the engine and openings that communicate with the advance and retard ports of the housing to allow oil to flow between the chambers.
To control the flow of fluid into and out of the spool, one opening of the spool is typically provided with a unidirectional valve such as a ball-valve or spring valve that permits flow of oil in one direction only, for example into the internal cavity of the spool, but not out of the internal cavity of the spool. The spool can be moved so that the valve is located at different ports, thereby controlling the direction flow of oil into and out of the ports.
However, such valves tend to be bulky, and add considerably to the overall size of the OCV, and/or reduce the flow capacity of the OCV.
It is also desirable in OCVs to isolate the engine oil supply from the oil in the spool. Oil in the OCV tends to become pressurised during use and high pressure oil could flow in a reverse direction back up the oil port into the engine, which would result in a loss of pressure, and hence diminishing the phase rate performance of the cam phasing system. Check valves can be integrated into the OCV to prevent this reverse flow of oil; however, these check valves are also bulky, and add to the size and weight of the OCV.
Against this background it is an object of the invention to address at least one of the problems associated with known OCVs.